This application is based upon and claims the benefit of Japanese Patent Applications No. 11-325473 filed on Nov. 16, 1999, and No. 2000-259116 filed on Aug. 29, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to mechanochemical polishing and more particularly, to a polishing technique utilizing chromium (III) oxide (Cr2O3) as abrasive grains.
2. Description of the Related Art
A power semiconductor device formed from a silicon carbide (SiC) having solid state properties with higher values compared to those of silicon (Si) has better performance than a power semiconductor device made of Si. Specifically, a SiC power semiconductor device can function as a semiconductor at high temperature because of its wide energy gap (which is about three times larger than that of Si), can withstand high voltage because of its high dielectric breakdown withstand voltage (which is ten times larger than that of Si), and can accommodate higher current because of excellent radiating properties attributable to its high coefficient of thermal conductivity (which is about three times larger than that of Si).
More specifically, for example, as shown in FIG. 22 a SiC wafer used to form an element is provided by epitaxially growing a SiC layer 101 having a low impurity concentration on a single crystal SiC substrate 100 having a high impurity concentration. An element 103 of a type that causes a current to flow in the longitudinal direction thereof (e.g., VDMOS) is formed on the SiC layer 101. While studies are being made into GaN as a material capable of emitting light having a short wavelength in the field of optical devices, SiC wafers are attracting attention as underlying substrates on which a GaN thin film is to be formed because Sic has less lattice mismatches with GaN compared to sapphire.
An SiC wafer is obtained by growing bulk single crystal SiC by sublimating SiC powder and re-crystallizing it on a seed crystal, cutting the bulk single crystal SiC into a configuration of a wafer and performing mirror finishing on a surface (cut surface) of the wafer. While a Sic layer or a GaN layer is epitaxially grown on the surface, the surface must be free of detects and must be smooth on the, level of atoms in order to obtain the epitaxial layer with high crystallinity.
A common method for mirror finishing of a SiC wafer is to achieve smoothness by polishing the surface using diamond abrasive grains. This is a method for mechanical surface finishing in which the surface of SiC (material to be polished) is polished using abrasive grains made of a material (diamond) harder than the surface. Although a smaller abrasive grain size results in a smoother surface (improved surface roughness), it cannot prevent the occurrence of defects (process-affected layer) originating from processing. Therefore, in this case, polishing is followed by a process for removing any process-affected layer as a post process such as dry etching or wet etching using hydrofluoric acid that is performed after growing an oxide film through thermal oxidation.
Further, diamond abrasive grains are expensive and become more expensive as the grain size is increased. Furthermore, when large abrasive grains are mixed in microscopic abrasive grains, surface roughness is not improved, and scratches may be produced to cause local deep defects. This results in a need for using high quality diamond abrasive grains with a uniform grain size, which also leads to an increase in the cost of abrasive grains. Process control (control of the abrasive grain size) is also difficult.
In a case of polishing using abrasive grains made of a material whose hardness is lower than a material to be polished, the processing can be performed with less damage on a processed surface. In this case, however, it is not possible to perform mechanical polishing in which the abrasive grains directly polish the material to be polished. It is preferable to adapt a polishing method in which a mechanically fragile reaction product (an oxide, compound or the like) is formed on the surface of the material to be polished and is peed off with soft abrasive grains, i.e., so-called mechanochemical polishing (chemical mechanical polishing; MCP). It is however difficult to form a reaction product on SiC because SiC is a chemically stable material.
With respect to mechanochemical polishing methods for SiC, there is a report on a method for polishing SiC using chromium oxide as abrasive grains (M. Kikuchi, Y. Takahashi, T. Suga, S. Suzuki, and Y. Bando, xe2x80x9cMechanochemical Polishing of Silicon Carbide Single Crystal with Chromium (III) Oxide Abrasivexe2x80x9d, J. Am. Ceram. Soc., 75[1] (1992) 189). According to the report, polishing (MCP) can be achieved without residual distortion and scratches by performing dry polishing of SiC on a disk (fixed abrasive grains) that is obtained by fixing chromium oxide abrasive grains with resin.
Further, Japanese Patent Laid-Open No. 7-80770 proposes a method in which surface flatness of SiC is improved by using chromium oxide power that is free abrasive grains, and a polishing platen made of a material having micro-Vicker""s hardness in the range from 1000 to 2000. Those methods are polishing methods that rely upon a mechanochemical phenomenon at contact points between a material to be polished and abrasive grains, and remove a reaction layer produced as a result of a direct solid phase reaction between them by a frictional action of the abrasive grains.
Such method utilizing a solid phase reaction was proposed in Japanese Patent Publication No. 56-23746, and the above-mentioned xe2x80x9cMechanochemical Polishing of Silicon Carbide Single Crystal with Chromium (III) Oxide Abrasivexe2x80x9d and Japanese Patent Laid-Open No. 7-80770 are based on a combination of SiC as a material to be polished and a chromium oxide as abrasive grains.
However, in order to cause the solid phase reaction, it is assumed that it is important to put the material to be polished and abrasive grains in contact with each other with a very high pressure at the contact points between them. Referring to the processing pressure, experiments were conducted under pressures of 0.34 MPa (3.5 kgf/cm2) in the above-mentioned xe2x80x9cMechanochemical Polishing of Silicon Carbide Single Crystal with Chromium (III) Oxide Abrasivexe2x80x9d, and under pressure of 900 kgf/cm2 in Japanese Patent Laid-Open No. 7-80770. When a high processing pressure is required and a wafer with a large diameter is polished, a polishing platen will be subjected to a very high pressure during polishing of the wafer. When several wafers are polished on the same polishing platen, a still higher pressure will be applied to the platen. Therefore, a polishing apparatus is required to have higher rigidity that is not experienced in the prior art.
Further, the processing pressure acts not only on a surface of a SiC wafer as contact points with abrasive grains but also on the wafer as a whole, which results in problems such as the occurrence of crystal distortion and defects, and the progress of existing defects. Furthermore, if uneven force is applied, the wafer might be cracked. On the other hand, when the processing pressures is low, no solid phase reaction occurs and the polishing time is prolonged by a low reaction speed or a low polishing speed even when a reaction occurs.
The present invention has been made under such limitations, and an object of the present invention is to provide a method and an apparatus for mechanochemical polishing, which make it possible to efficiently polish a hard material such as SiC at a low processing pressure.
According to the present invention, briefly, when a surface of a semiconductor wafer is polished using abrasive grains made of chromium (III) oxide, an oxidizing agent is supplied to exist on the surface of the semiconductor wafer to be polished. The abrasive grains made of chromium (III) oxide naturally serve as a catalyst for forming an oxide on the surface of the semiconductor wafer (for example, SiC), and the oxidizing agent further increases an amount of oxygen that reacts with the semiconductor wafer. As a result, polishing efficiency is improved. A hard material such as SiC can be polished efficiently even at a low processing pressure.